1. Technical Field
The present disclosure relates to an imaging system and a method for driving the imaging system.
2. Description of the Related Art
Addition of signals from a plurality of pixels is known in an application requiring high-speed signal reading such as a moving image. A photoelectric conversion apparatus having an amplifier in each column pixels provided in a matrix and configured to add a signal in a feedback capacitor included in the amplifier has been discussed in Japanese Patent Application Laid-Open No. 2008-78894.
The amplifier discussed in Japanese Patent Application Laid-Open No. 2008-78894 includes a clamp capacitor. The amplifier clamps signals output in response to the reset of the pixel to the clamp capacitor, and then sequentially adds signals output in response to photoelectric conversion. When the above-described clamp operation is performed, an input node of the amplifier is reset to a base potential. The output of the amplifier at this time includes an offset component resulting from the amplifier. Thereafter, a difference between the offset component and the signals after addition is obtained.
According to the operation of the photoelectric conversion apparatus discussed in Japanese Patent Application Laid-Open No. 2008-78894, in a process in which signals output from n (n is an integer of 2 or more) pixels are sequentially added, a level according to the reset of a first pixel is clamped with the clamp capacitor, and the signals from the n pixels are then added. More specifically, the signals from the n pixels are added while only a signal from one pixel is clamped.
For this reason, since the (n−1) offset components remain in the operation discussed in Japanese Patent Application Laid-Open No. 2008-78894, there is a concern that an S/N ratio of the signal output from the photoelectric conversion apparatus may be decreased.